One common method for printing images on a receiver member is referred to as electrography. In this method, an electrostatic image is formed on a dielectric member by uniformly charging the dielectric member and then discharging selected areas of the uniform charge to yield an image-wise electrostatic charge pattern. Such discharge is typically accomplished by exposing the uniformly charged dielectric member to actinic radiation provided by selectively activating particular light sources in an LED array or a laser device directed at the dielectric member. After the image-wise charge pattern is formed, the pigmented (or in some instances, non-pigmented) marking particles are given a charge, substantially opposite the charge pattern on the dielectric member and brought into the vicinity of the dielectric member so as to be attracted to the image-wise charge pattern to develop such pattern into a visible image.
Thereafter, a suitable receiver member (e.g., cut sheet of plain bond paper) is brought into juxtaposition with the marking particle developed image-wise charge pattern on the dielectric member. A suitable electric field is applied to transfer the marking particles to the receiver member in the image-wise pattern to form the desired print image on the receiver member. The receiver member is then removed from its operative association with the dielectric member and subjected to heat and/or pressure to permanently fix the marking particle print image to the receiver member. Of course, plural marking particle images of, for example, different color particles respectively can be overlaid on one receiver member (before fixing) to form a multi-color print image on the receiver member.
In the earlier days of electrographic printing, the marking particles were relatively large (e.g., on the order of 10-15 μm). As a result the print image had a tendency to exhibit a relief appearance (variably raised surface). Under most circumstances, the relief appearance was considered an objectionable artifact in the print image. In order to improve image quality, and to reduce relief appearance, over the years, smaller marking particles (e.g., on the order of less than 8 μm) have been formulated and are more commonly used today.
With the improved print image quality, print providers and customers alike have been looking at ways to expand the use of electrographically produced prints. In certain classes of printing, a tactile feel to the print is considered to be highly desirable. Specifically, ultra-high quality printing such as for stationary headers, business cards, or greeting cards and invitations, utilize raised letter printing to give a tactile feel to the resultant print output. This is currently carried out in the off-set industry via thermography in an off-line process. Some other instances where tactile feel in the print would be desirable are Braille prints or print documents, which have security features provided there within. Presently however, printing documents with raised information so as to have a tactile feel with electrographic techniques, particularly with commonly used small size marking particles, has not been practical, and past printing techniques where large particles were used produced low quality images.